\ usual, was quick i«> discern the most important charai t< i by which this 



family can bc differentiated from its allies. Ilis definition is as follows: 



"Corallum usually dichotomous or meur or less arborescent. Axis horn-like, or more or 



tlly ai base. Longitudinal ducts equal, arranged regularly all around the 



nchyma usually thick. Cells scattered over all parts of the surface, flat, or elevated 



on prominent \ rentacles at base, and sides of the polyps stiffened with large fusiform 



spieul nenchyma usually large, of various forms, must usually these are large 



i with clubs or crosses", 

 r the purposes of this work the following definition is otïered : 



enenchyma thick, without evident grooves; axis cylinder horny, or horny and calcareous, 

 but ointed. Primary water-vascular canals in a regular series around the axis. Calyces 



on all sides of the stem and branches, often entirely included. Spicules various, usually spindles, 

 clubs, butterfly-shaped or Blattkeulen. 



As Wright and Studer remark: "The genera belonging to the family as it now stands, 

 certainl) require revision". 



The material secured by the Siboga Expedition that can be referred to the Plexauridae 



is uot sufficiënt in extent to justify the author of this report in undertaking a n-vision of the 



family, and he therefore contents himself with recogfnizingf the eenera defmed in the Challeneer 



rt, and the addilion of a single new genus which seems necessary to accommodate a 



remarkable species collected by the Siboga Expedition. 



The most important feature in the diagnosis of genera in this family seems to be the 

 character of the axis cylinder, the forms of the calyces and spicules being however, of greal 

 service in generic description in some cases. 



The following artihcial key may be of service in aiding the student to determine the 

 generic affinities of species. 



Artificial key to the genera of Plexauruxe. 



i, branches and horny axis distinctly flattened Platygorgia) ' 



Calyces prominent and bilabiate (Eunicea 



Calyces moderately prominent, not bilabiate. 



Spicules armed with long, truncate verrucae (Anthoplexaurai 



Spicules small, the outer layer club-shaped iParaplexaura 



neither bilabiate nor decidedly prominent. Axis not distinctl) flattened. 

 Axis entirely horny, except at base, or with calcareous particles deposited 

 in the central core. 

 Spicules usually spindles. horizontally or longitudinally placed in outer 

 r of ccenenchyma. 

 nchyma thick. No very large, bar-like spicules Plexaura 



in tlit.- Sibi 



